1. Field of the Invention
This invention generally relates to a high speed magnetic levitation transportation system, and more particularly to an electromagnet for levitating and propelling a magnetic levitation vehicle along a guideway.
2. Description of the Related Art
Magnetic levitation (MAGLEV) transportation systems may be classified into two general categories based on the nature of their primary suspension systems: those employing an electromagnet suspension (EMS) system and those employing an electrodynamic suspension (EDS) system.
EMS-type MAGLEV systems levitate and propel a transport vehicle by inducing magnetic forces of attraction between vehicle-mounted electromagnets and ferromagnetic rails on a guideway. The German Transrapid is such a system. The electromagnets used in the German Transrapid have iron cores and nonsuperconducting copper coils. Excitation currents supplied to the coils induce a magnetic field in their respective iron cores. The poles of the cores, as a result, become attracted to the rails levitating the transport vehicle. U.S. Pat. Nos. 4,259,908, 4,953,470, and 5,152,227 disclose additional EMS-type MAGLEV systems which use iron-core electromagnets to levitate a transport vehicle.
There are a number of advantages and disadvantages associated with EMS-type MAGLEV systems such as the German Transrapid. One advantage is the existence of a low magnetic field in the passenger compartment of the transport vehicle. This is accomplished by confining most of the magnetic flux produced by the electromagnet to essentially a closed-loop path between the magnet core and the guideway rail. A second advantage is the ability of the transport vehicle to remain in a levitated state at slows speeds or at idle without a need for auxiliary retractable wheels. Additional advantages include low power consumption and resistance to derailing.
Many of the disadvantages associated with EMS-type MAGLEV systems which use non-superconducting electromagnets are centered around the comparatively low magnetic field that they generate. One significant disadvantage is that the clearance gap between the magnet core poles and the guideway rail is quite small, approximately 1 cm (0.4 inches) for example. The existence of a small clearance gap degrades system performance and jeopardizes operational safety by increasing the likelihood that the transport vehicle will become involved in an accident because of ice build up or other debris on the guideway. In addition, a system which operates with a small clearance gap is costly to construct and maintain. Other significant disadvantages include high vehicle weight and limited payload or freight capability.
EDS-type MAGLEV systems levitate and propel a transport vehicle by inducing magnetic forces of repulsion between vehicle-mounted electromagnets and ferromagnetic rails on a guideway. The Japanese MLU is such a system. The electromagnets used in the Japanese MLU are air-core superconducting electromagnets, which rely primarily on eddy currents to provide the levitation and propulsion forces required. Other EDS-type MAGLEV systems using air-core superconducting electromagnets are known, including, for example, one disclosed in U.S. Pat. No. 5,094,173.
The Japanese MLU overcomes some of the disadvantages associated with the German Transrapid. For example, the superconducting electromagnets used in the Japanese MLU produce a magnetic field which is much stronger than that produced by the copper coil electromagnets used in the German Transrapid. The Japanese MLU is therefore able to establish a significantly larger clearance gap (4-6 inches), permitting it to operate more safely, at a reduced cost, and with less maintenance than the German Transrapid.
The Japanese MLU, however, possesses at least three significant drawbacks which are not realized by the German Transrapid. First, the air-core of the Japanese MLU superconducting electromagnet is responsible for generating high levels of magnetic field in the transport vehicle. Shielding must be installed in order to compensate for this undesirable effect. Second, the Japanese MLU is susceptible to experiencing magnetic quench (i.e., changing from a superconducting state to a normal state) caused by dynamic effects. Third, the Japanese MLU must travel on wheels up to a speed of 60 miles/hour before it starts to levitate. In contrast, EMS vehicles remain levitated at all speeds, including stand still.
At least one MAGLEV system has been proposed which attempts to achieve the advantages of both the German Transrapid and Japanese MLU without achieving their disadvantages. This EMS-type MAGLEV system is disclosed in U.S. patent application Ser. No. 875,641, and represents a significant improvement over the German and Japanese systems, with its combination of an attractive-type suspension and an electromagnet having a superconducting coil and a multiple-pole iron core.
The advantages of the multiple-pole superconducting electromagnet include its ability to achieve a clearance gap with a relatively low number of ampere-turns, a reduction in the migration of stray magnetic fields into the transport vehicle, its ability to transmit most of the vehicle loading to the magnet core rather than to the coil windings, and a reduction in the possibility of magnetic quench caused by winding slip. However, this type of electromagnet requires a low-temperature superconducting coil which is costly to build, operate, and maintain. Also, electromagnets employing low-temperature superconducting coils have proven to be less reliable and demand stringent cryogenic cooling requirements, which necessarily requires the employ of a complex cryostat to maintain the coil in the superconducting state.